CAN a MiniDisc
Walkman rival the iPod? Sony thinks so. After exploiting some
intriguing physics to boost the storage capacity of the
languishing MiniDisc audio format more than fivefold, the
company thinks the technology can form the basis for a new
generation of cheap digital music players, digital cameras and
hand-held movie viewers.

Launched in
1992, a conventional MiniDisc (MD) records an hour of music or
speech on a 177-megabyte 6.5-centimetre disc. But although MD
has been a big success in Japan, it has been overshadowed
elsewhere by recordable CDs. But with its capacity now
expanded to 1 gigabyte per disc, Sony thinks it will be a
viable alternative to digital music players like Apple's
iPod.

The cheapest
iPod stores 1000 tracks on a 4-gigabyte hard drive and costs
$249. But Sony predicts the cheaper Hi-MD player it intends to
launch will store several hundred tracks on a new
6.5-centimetre disc. Anyone who wants more tracks simply slots
in a different disc - and these are likely to cost a dollar or
two at most.

Alternatively,
Sony says the new 1-gigabyte discs can hold over 6000
high-quality still photos, or five two-hour movies compressed
to the MPEG4 standard - a capacity that could lead to
Hi-MD-based digital cameras, or hand-held viewers for
displaying movies.

How did Sony
manage to quintuple the MD's capacity? When the system was
unveiled at the Consumer Electronics Show in Las Vegas two
weeks ago, the company refused to explain the technology. But
New Scientist has found the relevant patents and can
reveal how it works.

Unlike CD and
DVD, which are all-optical, an MD records data magnetically,
while a laser then reads it optically. To record, it heats a
spot of a ferromagnetic material with an infrared laser pulse
that briefly raises its temperature above the material's Curie
point - the temperature at which it loses its existing
magnetism. Then, as it cools, it becomes imprinted with a
magnetic field that is focused on it by a nearby magnetic
recording head. The recording is read by switching the laser
to a lower power, so that it does not heat the coating past
its Curie point, and optically sensing the effect each spot
has on the reflection of polarised light.

On a Hi-MD
disc, the magnetic domains are five times smaller than on an
ordinary MD, allowing it to record five times as much data.
These domains are too small to be read by the infrared
wavelengths used in MD players, because the beam simply cannot
focus tightly enough to read them, yet the wavelength cannot
be changed because Sony wants the Hi-MD players to be able to
play old MD discs.

Sony has come
up with a clever trick to resolve this dilemma. By employing
three magnetic layers instead of one, it has devised a way to
temporarily swell the shrunken domains to a readable size (see
Graphic). On top of the standard magnetic layer, a Hi-MD
disc has a "switching" layer, with a "displacement" layer on
top of that. When a domain isn't being read, the magnetic
field in the memory layer is coupled right through the
switching and displacement layers.

Crucially, the
middle switching layer is made from a magnetic material that
has a lower Curie point than the layer above. When the
read-out laser heats the disc, the spot on the switching layer
beneath the laser loses its magnetic properties, so it is no
longer coupled to the displacement layer above. This weakens
the "magnetic fence" around the domain in the displacement
layer so it temporarily swells to a readable size for a brief
moment as the laser passes. Sony's patents do not identify the
materials that do this.

Sony plans to
launch the Hi-MD players in the spring, alongside an online
music downloading service. Today's MD recorders sell for under
$100, with blank discs at $1. While Sony has not yet announced
the price of its new players and discs, it's hinting that they
will be far cheaper than most digital music players.